This invention relates to the production of relatively thick, low-stress films, particularly optical films, and to coated substrates produced.
Deposition of thin (less than 5 microns) electrically conductive films onto the support surface of an electrical non-conductor at non-elevated temperature conditions (not more than about 500 degrees C.) is known in the prior art. For example, in U.S. Pat. No. 2,769,778, a thin (0.05 micron), transparent electrically conductive metallic film is deposited onto the surface of a non-conductive material at a non-elevated temperature of 300-400degrees C. by cathode sputtering or thermal evaporation, respectively. Thick films, i.e. greater than a 5 micron thickness, are not contemplated using non-elevated temperature deposition techniques.
In another method at a non-elevated temperature (at 20-100 degrees C.), U.S. Pat. No. 3,320,484, a semiconductor integrated circuit is produced by depositing onto a support, a substantially planar thin layer (0.05-0.15 micron thickness) of aluminum vapor is deposited onto a high resistivity semiconductor wafer substrate, the thin dielectric layer including silicon nitride.
U.S. Pat. No. 3,463,715 relates to an elevated temperature (about 1000 degrees C.) method of sputter deposition of a semiconductor material such as silicon onto a substrate to produce a silicon layer having high conductivity. The deposition technique employed is cathode sputtering deposition.
Another elevated temperature method is provided in U.S. Pat. No. 3,560,364 for producing an extremely thin, free-standing or unsupported transparent film of silicon nitride having a low density, a low coefficient of thermal expansion, high flexibility and good dielectric properties. The sputtering deposition of the film onto a molybdenum substrate is carried out at a temperature of 500-900 degrees C., depending on the film thickness. The film thickness is in a range of 50 angstroms to 20 microns. As the thickness of the film increases, the operating temperature for producing that film moves upward toward 900 degrees, the upper end of the temperature range. The above technique described is known as RF sputtering.
A method of deposition is described in U.S. Pat. No. 3,600,218 in which insulating films of silicon nitride and aluminum nitride are deposited onto a substrate. Nitrogen is employed in the deposition reaction to form a thin, insulating film of silicon nitride or aluminum nitride, respectively, onto the substrate surface. The deposition technique described therein utilizes RF energy across a substrate and source to generate a plasma-containing source material at temperatures of 300 degrees C. or more. Again, the greater the film thickness, the higher the deposition temperature. At a film thickness of 0.5 microns, an operating temperature of 1250 degrees C. is indicated.
U.S. Pat. No. 3,629,088 covers a sputtering method for the deposition of silicon oxynitride. The low temperature deposition of silicon oxynitride onto an integrated circuit device is accomplished by the reactive sputtering of a thin layer (0.15 microns) of high-purity silicon source material in the presence of nitrous oxide and nitrogen.
A film of a transition metal silicide or an aluminum silicon alloy is deposited on a semiconductor substrate at a temperature of 500-1200 degrees C. by vacuum evaporation, and then used as an electrode or wiring of a semiconductor device. The thin film (0.3 microns in Examples) is produced by a sputtering method wherein the silicon component of the film was not supplied from the target, but from a gaseous silicon compound contained in the sputtering atmosphere. (See U.S. Pat. No. 4,218,291).
U.S. Pat. No. 4,656,101 describes an electronic device having a structure in which at least one electronic element, including an insulating film or covered with a protecting film, is formed on a substrate, at a temperature from ambient to 900 degrees C., where the insulating or protecting film can consist principally of aluminum nitride and a film consisting principally of silicon oxide or silicon nitride.
U.S. Pat. No. 4,711,821 relates to an optomagnetic recording medium which utilizes at least one thin film layer (0.01-0.02 microns) of silicon nitride. The underlying substrate can be a plastic material such as acrylic resin or polycarbonate.
U.S. Pat. No. 4,804,640 relates to a method of forming a three-region dielectric film, having an overall thickness of 0.01 microns, on silicon, and a semiconductor device employing such a film. The formation method includes the step of reactive sputtering of aluminum in an oxygen plasma atmosphere.
U.S. Pat. No. 4,846,948 shows a method of producing a an iron-silicon-aluminum alloy magnetic film of 1-20 microns thickness in which argon is entrapped by using a DC magnetron sputtering apparatus to apply RF bias to a substrate using an RF diode sputtering technique. Heat treatment of the alloy takes place at 450-800 degrees C.
A laminated composite and a method for forming same by chemical vapor deposition is the subject of U.S. Pat. No. 4,336,304. The composite includes a layer of opaque silicon aluminum oxynitride of a thickness between 12 and 50 microns, and an average of 25 microns, and a underlying substrate material to which the layer is bonded. The method includes the steps of exposing a surface of the material to an ammonia-containing atmosphere, heating the surface to at least 1200.degree. C., and impinging a gas containing in a flowing atmosphere of air, nitrogen, silicon tetrachloride, and aluminum trichloride on the surface. This is an example of a high temperature .deposition technique, typically at temperatures between 1000.degree. C. and 1400.degree. C., and at a pressure on the order of 240 megapascals (MPa).
Finally, in an article by Shiban K. Tiku and Gregory C. Smith in the IEEE Transactions on Electronic Devices, Volume Ed-31, No. 1, January, 1984, entitled, "Choice of Dielectrics for TFEL Displays", an attempt was made to deposit silicon aluminum nitroxide and Si.sub.3 N.sub.4 for use in AC thin-film electroluminescence displays. Films of 0.2-0.3 microns thickness were fabricated. The authors stated that thinner films of the dielectric material were found to be a problem because of pinholes, while thicker films caused high optical absorption and film stress. Since a high degree of optical transparency is required in many applications, such as in the electronics industry, the use of thick films is prohibited. Furthermore, high film stress results in a substantial amount and cracking and debonding of the film with respect to the underlying substrate, and in many cases resultant damage thereto.
Accordingly, there exists a need for a low-stress film material for coating underlying substrates, particularly silicon, silicon nitride and the like, by methods which are conducted at a non-elevated temperature (500.degree. C. or less), but which produce the above-described films at a relatively high thickness levels. These films should be transparent, hard, dense and mechanically stable even at thicknesses of 50 microns or greater.